1. Field of the Invention
The invention relates to subsea well installations; and, more particularly, to improvements in support means for well risers.
2. Description of the Prior Art
In the oil well industry, certain assemblies have been used for drilling oil wells in subsea locations. Generally, such assemblies include a riser carrying a string of drill pipe used to carry out the drilling operations. This riser extends from a vessel floating on the surface down to the subsea well. At the vessel, the upper portion of the riser extends through a throughbore in a rotary table which will subsequently be used to rotate the drill pipe during drilling operations. Such a riser is comprised of a plurality of sections and is assembled by supporting the uppermost section of the riser on the vessel, then subsequently adding sections and lowering the riser until the desired overall length is reached.
In such operations, a spider may be used which is disposed above the rotary table on the vessel and is opened to encircle the riser, then closed to support the riser sections by means of a flange on each riser section which flange is supported on the spider.
The riser sections are then added to the uppermost section of the riser supported on the spider by means of derricks or the like on the vessel which lifts a section over the assembled riser sections, then lowers it down onto the top of the riser where the flange of the added-on riser section may be bolted or otherwise secured to the flange of the topmost section of the assembled riser.
It is preferable during such operations that a constant load be maintained on the riser, regardless of motion of the vessel due to movement thereof. Further, if the added-on riser section comes down hard or drops down on top of the topmost riser section, a tremendous load is placed on the assembled riser which itself is of considerable weight. If this load is slightly off, it will drop one side of the spider. This may cause considerable damage to both the riser, and/or the rotary table or vessel itself.
There is thus a need for means maintaining a constant balanced load on a riser extending from a vessel to a subsea wellhead installation, and particularly a need for compensating for loads placed on the supporting equipment on the vessel as sections or buoyancy chambers or the like are added onto the riser.